Field emission electron gun device of the automatic control type

ABSTRACT

A field emission electron gun device wherein the quantity of electrons in the electron beam is automatically controlled by means of a control electrode disposed adjacent the cathode. The control electrode is disposed behind the cathode tip in the direction of beam projection so that the electron optical characteristics will not deteriorate although the voltage applied to the control electrode varies.

United States Patent [191 Nomura et al.

[4 1 Dec. 9, 1975 FIELD EMISSION ELECTRON GUN DEVICE OF THE AUTOMATIC CONTROL TYPE [75] Inventors: Setsuo Nomura; Tsutomu Komoda;

Yasushi Nakaizumi, all of Katsuta,

Japan I [73] Assignee: Hitachi, Ltd., Japan [22] Filed: Oct. 29, 1973 [21] Appl. No.: 410,444

[30] Foreign Application Priority Data Feb. 23, 1973 Japan 48-21287 [52] U.S. Cl. 315/383; 315/30; 315/381 [51] Int. Cl. H01J 29/70 [58] Field of Search 315/30, 31, 29, 379, 381,

[56] References Cited UNITED STATES PATENTS 3,659,135 4/1972 Hendrikus 315/31 R 3,678,333 7/1972 Coates 315/31 R 3,760,383 9/1973 Wolfe et a]. 3,786,305 1/1974 Komoda et a1. 315/30 Primary Examiner-Maynard R. Wilbur Assistant Examiner-J. M. Potenza Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT A field emission electron gun device wherein the quantity of electrons in the electron beam is automatically controlled by means of a control electrode disposed adjacent the cathode. The control electrode is disposed behind the cathode tip in the direction of beam projection so that the electron optical characteristics will not deteriorate although the voltage applied to the control electrode varies.

15 Claims, 3 Drawing Figures US. Patent Dec. 9, 1975 FIG. 2

FIG. 3

2 3 Ve (KV) FIELD EMISSION ELECTRON GUN DEVICE OF THE AUTOMATIC CONTROL TYPE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field emission electron gun, and more particularly to a field emission electron gun wherein the emission cur-rent is automatically controlled.

2. Description of the Prior Art The ordinary field emission electron gun has several disadvantages, one of which relates to the fact that the stability of the generated electrons is not as good as that provided by a thermionic emission gun.

The quantity of electrons emitted due to the force of an electrostatic field is a function of the intensity of the electrostatic field (F) which acts on the surface of the cathode. Thus, automatic changes in the intensity of the electrostatic field corresponding to the changes of the quantity of electrons forming the electron beam will be effective to stabilize the electron beam. Accordingly, an emission current control electrode may be added to an ordinary field emission electrode gun which comprises a cathode, a first anode and a second anode, and the potential of the emission current control electrode (Va) is controlled automatically with respect to the changes of the quantity of electrons in the electron beam (le). That is, the magnitude of the beam current Ie is detected, and the detected value is negatively fed back to the emission current control electrode by a negative feed back control circuit so that the magnitude of the potential of the control electrode Ve is automatically controlled in order to maintain the magnitude of the electron beam current le at a predetermined value. Hereinafter, the electron gun is referred to as a field emission electron gun of the automatic control type, by which a very stable electron beam may be obtained.

However, this type of electron gun has certain disadvantages with respect to the electron optical characteristics thereof. That is, the attachment of an emission current control electrode has the same effect as a weak electrostatic lens disposed between the cathode and the first anode. In addition, since the potential of the control electrode Ve, whichalso determines the focal distance fe of the lens, continuously varies so as to maintain the magnitude of the electron beam current Ie at a predetermined value, the electron optical characteristics (i.e., the position of the convergence point of the electron beam) varies continuously because the changes in the potential Ve cause changes in the focal distance fe. As a result of this, this type of electron gun fails to provide a sufficient convergence of the electron beam in a practical device such as an electron microscope of the scanning type so that it may not be usable in such a device where a very fine electron beam is required.

SUMMARY OF THE INVENTION The object of the present invention is to provide a field emission electron gun of the emission current automatic control type, in'which the electron optical emission electron gun in such a manner that the surface of the first anode opposed to the cathode is semispherical with the tip of the cathode at the geometric center thereof and the emission current control elec- 5 trode is disposed more adjacent to the cathode body than the tip of the cathode (opposed-side of the first anode as seen from the tip of cathode).

These and other features of the present invention will become apparent when considered with the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an embodiment of a field emission electron gun of the automatic control -'type in accordance with the present invention;

FIG. 2 is a graph showing the functional relationship between f0 and Ve when an emission current control electrode is disposed in front of the tip of the cathode and at the rear thereof; and

FIG. 3 is a graph showing the functional relationship between f0 and Ve when the surface of a first anode opposed to the cathode is cut flat and semi-spherically.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings. Referring now particularly to FIG. 1, there is generally shown an example of a field emission electron gun system of the automatic control type, which comprises a cathode l 1, an annularly or cylindrically shaped emission current control electrode 12, a first anode 13, an accelerating anode l4 and a specimen 15. The control electrode 12 is disposed at the rear portion behind the tip of the cathode 11. The anode 13 is cut in such a manner that it has a semi-spherical or hemispherical surface on the side thereof facing the cathode 11. An accelerating voltage generating source 16 is connected to anode 13 and a cathode heating power source 17 is connected in series with source 16 to the cathode 11.

The electron beam which is directed to the specimen 15 is detected by a detecting known device 20, from which output signals are fed to a control device 19 to adjust the control potential level Ve of the control voltage source 18 to a proper value.

The detecting device 20 measures the value of the beam current derived from the specimen 15 by way of a resistor 21 connected between the specimen and ground. The potential across resistor 21 is measured by an amplifier 23 connected via a variable capacitor 22 to the resistor 21 so that variations in the current through resistor 21 produce a signal output from amplifier 23, which signal is applied in control of voltage source 18. The operation by which the voltage Ve of the emission current control source 18 is automatically 'controlled in order to maintain the magnitude of the electron beam current to a given value is similar to that of the aforementioned field emission electron gun of the automatic control type. It is preferable that changes in the electron optical characteristics of the electron beam corresponding to the changes in voltage Ve (Ave) be as small as possible. It has been suggested that the electron optical characteristic of the electron beam is represented by the convergence position (f0) of the electron beam. Therefore, the changes in the electron optical characteristic of the electron beam can be reduced by decreasing the changes info (Afo) corresponding to A Ve.

The present inventors study shows the fact that the magnitude Afo/AVe correlates with the shapes and the arrangement of the emission current control electrode 12 and the first anode 13. FIG. 2 shows the measurements of the functional relationship between f and Ve when the emission current control electrode 12 is displaced. FIG. 3 shows the measurements of the functional relationship between f0 and Ve when the first anode is changed in shape.

In FIGS. 2 and 3 f0 represents the imaginary convergence position (imaginary light source position) of the electron beam at the side of the cathode 11, and its magnitude indicates the value measured in the direction from the tip of cathode 11 to the opposed side of the first anode 13. In both FIGS. 2 and 3 the length between the cathode 11 and the first anode is 33mm, the voltage across the cathode l1 and the first anode is 3KV. In FIG. 2 the curve 100 shows the functional relationship between f0 and Ve when the emission current control electrode 12 is placed in front of the cathode tip, and the curve 200 shows the functional relationship between f0 and Ve when it is placed at the rear of the cathode tip as seen in FIG. 1. In FIG. 3 the curve 101 shows the functional relationship between f0 and Ve when the surface of the first anode 13 is flat on the side facing cathode 11. A curve 201 shows the functional relationship between f0 and Ve when the surface of anode 13 facing cathode l 1 is semi-spherical with the cathode tip positioned at the geometric center thereof.

It is seen from FIG. 2 that the value Afo/AVe can be reduced by providing the emission current control electrode 12 at the rear of the tip of cathode 11. And it is also seen from FIG. 3 that the value Afo/AVe can be reduced by forming the surface of anode 13 facing cathode 11 to be semi-spherical with the cathode tip at the geometric center thereof. For example, the value of Afo/AVe when the surface of the first anode 13 is semispherical becomes less than percent of that when the surface is flat. As a result of these improvements, the variations in the convergence position of the electron beam which essentially existed in the prior field emission electron gun of the emission curret automatic control type is decreased by more than 80 percent by adopting the features of the present invention.

The magnitude of the variation is the same as the effect due to chromatic aberration which existed in the ordinary field emission electron gun. In accordance with the present invention, a very fine electron beam may be maintained. That is, a very fine and stable electron beam can be obtained without deteriorating the electronic optical characteristics by means of the field emission electron gun of the automatic control type, as provided in accordance with the present invention. The purpose of the invention may be sufficiently accomplished by either providing the emission current control electrode 12 at the rear of cathode tip or cutting the surface of the first anode 13 in a semi-spherical shape, or both methods may be carried out together.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty.

What is claimed is:

1. In an electron gun device of the field emission type for generating an electron beam having a disposed along a beam path a cathode having an electron emitting surface, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement comprising a substantially cylindrically shaped control electrode the entire length of which is disposed behind the electron emitting surface of said cathode with respect to the direction of electron emission and voltage control means responsive to said detection means for applying a variable voltage to said control electrode whereby the field emission current from said cathode is controlled in accordance with the output from said detecting means.

2. An electron gun device as defined in claim 1 wherein said first anode has a semispherical surface facing said cathode with said cathode being disposed at the geometric center thereof.

3. An electron gun device as defined in claim 1 wherein said cathode is a point source cathode and said control electrode surrounds the body of said cathode behind the cathode tip.

4. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement comprising a control electrode disposed in the vicinity of said cathode and voltage control means responsive to said detection means for applying a variable voltage to said control electrode, whereby the field emission current from said cathode is controlled in accordance with the output from said detecting means, said first anode having a hemispherical surface facing said cathode with said cathode being disposed at the geometric center thereof.

5. An electron gun device as defined in claim 4 wherein said cathode is a point source cathode and said control electrode is an annular electrode surrounding the body of said cathode behind the cathode tip.

6. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement wherein said first anode has a hemispherical surface facing said cathode with said cathode being disposed at the geometric center thereof.

7. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path, in sequential order, a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode, so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement wherein said first anode has a surface which faces said cathode, said surface being non-linearly sloped.

8. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode, so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement wherein said first anode has a concave surface which faces said cathode.

9. The improvement according to claim 8, wherein said concave surface of said first anode is disposed symmetrically with respect to said beam path.

10. The improvement according to claim 8, wherein said concave surface is a hemispherical surface.

11. The improvement according to claim 8, further comprising a control electrode disposed in the vicinity 6 of said cathode and voltage control means, responsive to said detection means, for applying a variable voltage to said control electrode.

12. The improvement according to claim 11, wherein said cathode is a point source cathode and said control electrode is a cylindrically shaped electrode surrounding the body of said cathode behind said cathode tip.

13. The improvement according to claim 12, wherein said concave surface of said first anode is disposed symmetrically with respect to said beam path.

14. The improvement according to claim 12, wherein the entire length of said cylindrically shaped control electrode is disposed behind the electron emitting surface of said cathode.

15. The improvement according to claim 8, wherein said cathode, said first anode and said second anode are disposed in sequential order along said beam path. 

1. In an electron gun device of the field emission type for generating an electron beam having a disposed along a beam path a cathode having an electron emitting surface, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement comprising a substantially cylindrically shaped control electrode the entire length of which is disposed behind the electron emitting surface of said cathode with respect To the direction of electron emission and voltage control means responsive to said detection means for applying a variable voltage to said control electrode whereby the field emission current from said cathode is controlled in accordance with the output from said detecting means.
 2. An electron gun device as defined in claim 1 wherein said first anode has a semispherical surface facing said cathode with said cathode being disposed at the geometric center thereof.
 3. An electron gun device as defined in claim 1 wherein said cathode is a point source cathode and said control electrode surrounds the body of said cathode behind the cathode tip.
 4. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement comprising a control electrode disposed in the vicinity of said cathode and voltage control means responsive to said detection means for applying a variable voltage to said control electrode, whereby the field emission current from said cathode is controlled in accordance with the output from said detecting means, said first anode having a hemispherical surface facing said cathode with said cathode being disposed at the geometric center thereof.
 5. An electron gun device as defined in claim 4 wherein said cathode is a point source cathode and said control electrode is an annular electrode surrounding the body of said cathode behind the cathode tip.
 6. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement wherein said first anode has a hemispherical surface facing said cathode with said cathode being disposed at the geometric center thereof.
 7. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path, in sequential order, a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode, so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement wherein said first anode has a surface which faces said cathode, said surface being non-linearly sloped.
 8. In an electron gun device of the field emission type for generating an electron beam having disposed along a beam path a cathode, a first anode and a second anode, and further including means for applying an electric voltage between said cathode and said first anode, so as to produce an electric field therebetween which causes electrons to be emitted from said cathode toward said first and second anodes, and means for detecting the electron beam current, the improvement wherein said first anode has a concave surface which faces said cathode.
 9. The improvement according to claim 8, wherein said concave surface of said first anode is disposed symmetrically with respect to said beam path.
 10. The improvement according to claim 8, wherein said concave surface is a hemispherical surface.
 11. The improvement according to claim 8, further comprising a control electrode disposed in the vicinity of said cathode and voltage control means, responsive to said detection means, for applying a variable voltage To said control electrode.
 12. The improvement according to claim 11, wherein said cathode is a point source cathode and said control electrode is a cylindrically shaped electrode surrounding the body of said cathode behind said cathode tip.
 13. The improvement according to claim 12, wherein said concave surface of said first anode is disposed symmetrically with respect to said beam path.
 14. The improvement according to claim 12, wherein the entire length of said cylindrically shaped control electrode is disposed behind the electron emitting surface of said cathode.
 15. The improvement according to claim 8, wherein said cathode, said first anode and said second anode are disposed in sequential order along said beam path. 